Heat stabilization of polyamides



time.

'polyamines'.

United States Patent 3,174,944 HEAT STABILIZATION 0F POLYAMIDES .liirgen Brandes, Wiesbaden, Germany, assignor to Vereinigte Glanzstoif-Fabriken AG., Wuppertal=Elberfeld,

Germany No Drawing. Fiied Jan. 13, 1960, Ser. No. 2,127 Claims priority, applicatiimlggrmany, Jan. 14, 1959,

12 Claims. (Cl. 260-417) This invention relates to}. the heat stabilization of polyamides, and in particular to an improvement in the heat stability of a polyamide product.

It is well known that polyamides become damaged when subjected to elevated temperatures over a period of For example, the tensile strength and elongation properties of filaments and the like become progressively poorer under the action of heat. For this reason, many heat stabilizing additives to polyamides have been suggested in order to provide a product with improved heat stability. Some of these additives are added to the polyamide prior to its extrusion or spinning and others are incorporated by treatment subsequent to extrusion or spinning.

Among heat stabilizing additives which have been used are the N-aryl-substituted secondary aromatic diamines or Other compounds described in the art are: manganese compounds, N,N' polymethylene bis ohydroxbenzamide, Z-mercaptobenzimidole, and copper salts which have an ionizable or exchangeable hydrogen atom. The copper salts have also been used in combination with halides or with phosphoric acid and halides.

An improvement in heat stability can also be achieved by treating the polyamide with hypophosphoric acid or hypophosphites. These compounds, however, can only be added to the polyamide after polymerization of the polyamide from its monomeric reactants because hypophosphoric acid and its salts cannot withstand the polymerization temperature.

The known stabilizing materials vary quite extensively in their effectiveness. It is believed that the best results have been achieved with the copper salts, especially in combination with alkali halides. The iodides are preferred for this purpose or may also be used alone. However, the copper salts have certain disadvantages. For example, these salts severely discolor the polyamide product, and to a large extent they are washed out from the polyamide during extraction of the cuttings of the product during its normal processing. The tendency to wash out of the product is especially noticeable with alkali iodides which only have the advantage of being colorless. Accordingly, the iodides such as potassium iodide are particularly disadvantageous because very large amounts must be added in order to obtain an effective stabilization. In our experiments, we have found that the content of elemental iodine in the final product should be about 5.O by weight with reference to the polyamide, whereas the same stabilizing effect can be achieved with a copper content of only about 5.0 10 by weight. The greater amount of iodine would not be a difiicult problem except that to provide the final product with the minimum effective iodine content, a substantially larger quantity must be added at an earlier stage of the process. Furthermore, when the inorganic iodides employed by the prior art are washed out, it is not possible to recover them, and the loss of large amounts of these additives increases the ultimate cost of the final product.

Another disadvantage has been found if potassium iodide is added prior to polymerization of the polyamide from its initial monomeric reactants. An unfavorable effect is produced, apparently due to the ions of the iodide in solution. Difficulties in spinning and stretching 3,174,944 Patented Mar. 23, 1965 "ice the polyamide are especially observed with a polyamide which is matted or given a dull finish with titanium dioxide. For example, when a dispersion of the titanium dioxide comes in contact with a potassium iodide solution, the titanium dioxide agglomerates or flocks, and this phenomena leads to an uneven distribution of the titanium dioxide in the polyamide such that spinning and stretching are quite difficult.

From the foregoing discussion, it will be observed that while iodine does not discolor the polyamide as does copper and has relatively good heat-stabilizing properties, the use of iodine-containing additives has not been particularly useful in this art because of the above-mentioned disadvantages.

One object of the present invention is to provide a polyamide with improved properties of heat stabilization.

Another object of the invention is to provide an improved iodine-containing heat stabilizing additive for polyamides.

Still another object of the invention is to provide a process in which the heat stabilizing compound can be added prior to polymerization and still produce a polyamide product wherein the stabilizer is relatively unaffected by the polymerization itself or by subsequent washing operations.

Yet another object of the invention is to provide a heat stabilizer for polyamides whereby the stabilizing compound is compatible with the addition of titanium dioxide dispersions which are commonly incorporated during production of the polyamide.

Other objects and advantages of the invention will become more apparent in the following detailed description of the invention.

In accordance with the invention, it has now been found that an improved polyamide product can be obtained by adding to the monomeric reactant material, i.e. prior to polymerization, an iodobenzoic acid in a small quantity sufiicient to give a heat stabilized final product. The term polyamide is employed herein with special reference to polyhexamethylene adipamide, i.e. nylon, and polycaprolactam, these products having an extensive commercial use. However, these products may be slightly modified in known manner to obtain a variety of properties without changing the essential polyamide structure, and such modified products should be considered equivalents within the scope of this invention. The term iodobenzoic acid is employed herein and in the appending claims with reference to benzoic acid containing a single iodine substitutent in either the ortho, para or meta position of the benzene ring relative to the carboxy substituent, i.e. a compound of the formula The final polyamide product should have an iodine content of at least about 0.05% by weight up to about 0.25% by weight and preferably about 0.05 to 0.1% by weight. Since it is often impossible to avoid some Washing out of the stabilizer, the iodobenzoic acid should be added prior to polymerization with an amount of iodine of about 2.5 to 3.5 times that which is required in the final product. In other words, the iodobenzoic acid should be added to the monomeric reactant material in a quantity of about 0.2 to 1.2% by weight and preferably about 0.2 to 0.8% by weight with reference to the weight of the resulting polyamide. As regards polycaprolactam, the quantity of iodobenzoic acid added prior to polymerization can be expressed in terms of molar percent with respect to the number of mols of the caprolactam reactant and is preferably about 0.1 to 0.5 mol percent.

The process and product of the invention are especially advantageous because the iodobenzoic acid appears to have a double function of acting as a chain breaker and also as a heat stabilizer which is more securely bound to the polyarnide and cannot be easily washed out of the product. The polyarnide is not discolored by the stabilizer of the invention and does not cause trouble when used in combination with titanium dioxide. Loss of the stabilizer is not excessive when the polyamide is subjected to the usual washing or extraction procedures.

The invention is further illustrated by the following examples wherein percentages are expressed by weight with respect to the polyamide product unless otherwise indicated to be a molar percent based upon a monomeric reactant. The examples are illustrative only and are not intended as limiting the scope of the invention.

1. PRODUCTION OF FILAMENTS Example 1 20 kilograms of caprolactarn, 2 kilograms of Water and 0.2 mole percent p-iodobenzoic acid with reference to the lactam are polymerized in a SO-liter pressure autoclave which is heated with diphenyl, first for one hour at 18 atmospheres above normal pressure, i.e. 19 atmospheres, and 270-290" C. and then for 1 /2 hours more with pressure falling from 19 atmospheres down to atmospheric pressure while maintaining the same temperature of 270-290 C. With the aid of nitrogen, the resulting polyamide is extruded from the autoclave in the form of a continuous band. The band is reduced to cuttings and these are extracted by boiling with water until the extractable content of the cuttings is less than 1%. The solution viscosity of the cuttings amounts to 2.16. After drying, the cuttings are spun under normal conditions to a denier of 100/30.

Example 2 The same procedure is followed as in Example 1, but with the addition of 0.285 mol percent p-iodobenzoic acid with respect to the caprolactam. The solution viscosity of the cuttings amounts to 2.00.

Example 3 The same procedure is followed as in Example 1, but with addition of 0.3 mol percent o-iodobenzoic acid with reference to the caprolactam. The solvent viscosity of the cuttings amounts to 1.99.

Example 4 Under the same experimental conditions as given in Example 1, a anatt or dull finish polymerizate is pro duced from 20 kilograms of caprolactam, 2 kilograms of water, 0.2 mol percent m-iodobenzoic acid with respect to the lactam and 60 grams of titanium dioxide (anatase) as a dispersion stabilized with sodium hexametaphosphate. The solvent viscosity of the cuttings amounts to 2.15.

Example 5 Following the same experimental conditions of Example 1, a comparative sample was prepared .by using kilograms of caprolactam, 2 kilograms of water and 0.2 mol percent benzoic acid in place of the iodobenzoic acid.

II. SAMPLE TESTING .ples.

TABLE 1 [Heating for 1 hour at 0.]

Quantity Tensile Elonga- Sample Stabilizer Added, Strength tion Mol Loss in Loss in Percent Percent Percent p-Iodobenzoic acid 0. 2 None 28 .do 0.285 None 20 o-Iodobenzoic aci 0.3 None 26 m-lodobenzoic acid 0. 2 None 28 Benzoic acid 0. 2 32 63 TABLE 2 [Heating for 1 hour at 0.]

Quantity Tensile Elonga- Sample Stabilizer Added, Strength tion Mol Loss in Loss in Percent Percent Percent p-Iodobenzoic acid 0. 2 9.5 39 O 0.285 None 35 o-Iodobenzoic acid 0.3 None 28 4 m-I0dobenzoic acid O. 2 6. 0 32 5 Benzoic acid 0. 2 56 77 TABLE 3 [Heating for 1 hour at, 0.]

Quantity Tensile Elonga- Sample Stabilizer Added, Strength tion Mol Loss in Loss in Percent Percent Percent p-Iodobenzoic acid 0. 2 12.0 49 do 0. 285 4. 0 37 o-Iodobenzoic aci 0.3 None 30 m-Iodobenzoic acid. 0. 2 8. 0 36 Benzoic acid 0. 2 68 80 Before After washing, washing, percent percent Example 1 7. 0X10- Example 2 9. 0X10" Example 3 9. 5X10- Example 4 6. 8X10- By comparison, if the same polycaprolactam isproduced under the conditions of Example 1, but with the incorporation of an iodine content in the form of potassium iodide instead of the iodobenzoic acid, an iodine content of 2.5 l0- prior to extraction of the cuttings is reduced to a content of only 10 No improvement in the heat stability of the final product could be obtained.

It is quite obvious from the foregoing description that an exceptionally good heat stability of polycaprolactam is obtained by using iodobenzoic acid as the stabilizing agent. Substantially the same results are obtained in similar tests with addition of the iodobenzoic acid to nylon.

Example 6 20 kilograms of adipic hexamethylene diamine, 2 kilograms of water and 0.3% by weight p-iodobenzoic acid with reference to the adipic hexarnethylene diamine are condensed in a 50-liter pressure autoclave which is heated with diphenyl, first for 1% hours at 18 atm. and 270 290 C., and then for 2% hours more with pressure falling from 18 atm. down to atmospheric pressure while maintaining the same temperature of 270290 C. With the aid of nitrogen, the resulting polyamide is extruded from the autoclave in the form of a continuous band.

The band is reduced to Cuttings and these are spun under normal conditions to a denier of 100/30.

The invention is hereby claimed as follows:

1. In a method of heat stabilizing a polyamide selected from the group consisting of polyhexamethylene adipamide and polycaprolactam wherein a stabilizing agent is incorporated prior to polymerization into the monomeric reactant material from which the polyamide is produced, the improvement which comprises employing iodobenzoic acid of the formula as the stabilizing agent in a quantity sutiicient to provide a heat-stabilized polyamide product.

2. The improved method as claimed in claim 1 wherein the stabilizing agent is o-iodobenzoic acid.

3. The improved method as claimed in claim 1 wherein the stabilizing agent is m-iodobenzoic acid.

4. The improved method as claimed in claim 1 wherein the stabilizing agent is p-iodobenzoic acid.

5. A composition of improved heat stability comprising a polyamide selected from the group consisting of polyhexamethylene adipamide and polycarprolactam to which there has been added iodobenzoic acid of the formula as a heat stabilizing agent such that the polyamide has an iodine content of at least about 0.05% by weight.

6. A composition as claimed in claim 5 wherein the heat stabilizing agent is o-iodobenzoic acid.

7. A composition as claimed in claim 5 wherein the heat stabilizing agent is m-iodobenzoic acid.

8. A composition as claimed in claim 5 wherein the heat stabilizing agent is p-iodobenzoic acid.

9. A polyamide composition of improved heat stability as claimed in claim 5 which contains a finely dispersed titanium dioxide.

10. A composition of improved heat stability comprising a polyamide selected from the group consisting of polyhexamethylene adipamide and polycaprolactam to which there has been added iodobenzoic acid of the formula as a heat stabilizing agent such that the polyamide has an iodine content of between about 0.05% by weight and 0.25% by weight.

11. A composition of improved heat stability consisting essentially of polycaprolactam to which there has been added iodobenzoic acid of the formula as a heat stabilizing agent such that said polycaprolactam has an iodine content of between about 0.05 and 0.25 by weight.

12. A composition of improved heat stability consisting essentially of polyhexamethylene adipamide to which there has been added iodobenzoic acid of the formula as a heat stabilizing agent such that said polyhexamethylene adipamide has an iodine content of between about 0.05 and 0.25% by weight.

References (Cited in the file of this patent (1955), copy in Div. cf. C.A., 49, 14372d (1955).

Whitmore: Organic Chemistry, 691-696, 2nd edition, D. Van Nostrand (10., Inc., N.Y., 1951.

Hine: Physical Organic Chemistry, 80, McGraw- Hill Book Co., Inc., N.Y., 1956. 

5. A COMPOSITION OF IMPROVED HEAT STABILITY COMPRISING A POLYAMIDE SELECTED FROM THE GROUP CONSISTING OF POLYHEXAMETHYLENE ADIPAMIDE AND POLYCARPROLACTAM TO WHICH THERE HAS BEEN ADDED IODOBENZOIC ACID OF THE FORMULA 